There was a nuclear foreign policy based on deterrence called M.A.D. or Mutually Assured Destruction. If both side were assured that the outcome would be deadly to the countries in and around the zone of bombing, then they wouldn't do it, which provide to be very affective. So far, no threat of using the Atomic or Hydrogen bomb has been made in between the USSR and America.
I hope this helps! Sorry if it doesn't!